High level non-contacting dynamic voltage follower for voltage measurement of electrostatically charged surfaces

ABSTRACT

A wide band voltage follower circuit for voltage measurement of electrostatically charged surfaces through the use of a probe having a capacitor detector arranged in spaced and non-contacting manner with the surface under measurement to detect the unknown surface voltage thereof, the probe having a cable connected to the output of the capacitor detector. An ultra high impedance low level voltage follower is connected to the capacitor detector output through the cable, and shield means driven by the output of the ultra high impedance low level voltage follower are employed to shield the latter, the cable, and the probe, thereby minimizing the effective input capacitance of the ultra high impedance low level voltage follower. A high level voltage follower is connected to the output of the ultra high impedance low level voltage follower, and a floating power supply is connected to power the ultra high impedance low level voltage follower. The power supply is bootstrapped with the latter by the high level voltage follower through the connection of the common line of the floating power supply to the output of the high level voltage follower. The output of the high level voltage follower is connected to a peak-to-peak detector to derive an output voltage whose D.C. voltage is equal to the D.C. voltage of the surface under measurement.

United States Pate Vosteen HEGH LEVEL NON-CONTACTING DYNAMIC VOLTAGEFOLLOWER FOR VOLTAGE MEASUREMENT OF ELECTROSTATICALLY CHARGED SURFACESRobert E. Vosteen, 315 West Center Street, Medina, N.Y. 14103 22 Filed:Feb. 13,1970

21 Appl.No.: 11,075

[76] Inventor:

UNITED STATES PATENTS Kim ..324/32 Miller ..324/l23 PrimaryExaminerMichael J. Lynch Attorneylrons, Stockman, Sears & Santorelli UHZLOW LEVEL VOLTAGE FOLLOWER probe,

[4 1 Apr. 24, 1973 [57] ABSTRACT the use of a probe having a capacitordetector ar-.

ranged in spaced and non-contacting manner with the surface undermeasurement to detect the unknown surface voltage thereof, the probehaving a cable connected to the output of the capacitor detector. Anultra high impedance low level voltage follower is connected to thecapacitor detector output through the cable, and shield means driven bythe output of the ultra high impedance low level voltage follower areemployed to shield the latter, the cable, and the thereby minimizing theeffective input capacitance of the ultra high impedance low levelvoltage follower. A high level voltage follower is connected to theoutput of the ultra high impedance low level voltage follower, and afloating power supply is connected to power the ultra high impedance lowlevel voltage follower. The power supply is bootstrapped with the latterby the high level voltage follower through the connection of the commonline of the floating power supply to the output of the high 10 Claims, 6Drawing Figures HIGH LEVEL VOLTAGE FOLLOWER PEAK. T0 PEAK /J /\DRWENDETECTOR 1 2 OUTPUT J L V ELECTROSTATIC SHIELD :%:CSP l A I/DRIVENELECTROSTATIC SHIELD d l SURFACE 1 LINE UNDER TEST l FLOATINGBOOTSTRAPPED L POWER SUPPLY Patented April 24, 1973 3,7295675 2Sheets-Sheet l FIG. I

' AZI%- GROUNDED BAND Fla-2b |L Y l OUTPUT 3 DI C3 I 1 UHZ LOW LEVEL (A41 1- HIGH LEVEL VOLTAGE FOLLOWER VOLTAGE EOLLOWER J PEAK TO PEAKDETECTOR 1 2 OUTPUT DRIVEN ELECTROSTATIC SHIELD DRIVEN ELECTROSTATICSHIELD sum E 3TH C I UNDER TEST INVENTOR ,ROBERT E. VOSTEEN FLOATINGBOOTSTRAPPED I r, ,1 j

POWER SUPPLY BY W I/jk'IY'ICl/A s a 1 I QE'CULELA ATTORNEYS PatentedApril 24, 1973 2 Sheets-Sheet 2 OUT- PUT PEAK TO PEAK DETECTOR 1 *0+ H,V. SUPPLY VOLTAGE FOLLOWER UHZ LOW LEVEL DRIVEN 5mm) VII/I IJII INVENTORROBERT E, VOSTEEN ATTORNEYS HIGH LEVEL NON-CONTACTING DYNAMIC VOLTAGEFOLLOWER FOR VOLTAGE MEASUREMENT OF ELECTROSTATICALLY CHARGED SURFACESBRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram illustratingan operating principle of the invention;

FIGS. 2a and 2b are schematic diagrams more fully showing how theprinciple illustrated in FIG. 1 is applied to the invention;

FIG. 3 is a block diagram illustrating the invention;

FIG. 4 is a sectional view of a probe that may be used according to theinvention;

FIG. 5 is an electrical schematic diagram showing in detail a circuitwhich may be used to practice the invention.

GENERAL DESCRIPTION OF THE INVENTION It is known that dynamic voltagemeasurement of a moving electrostatically charged surface can be made byplacing a probe at a fixed distance from the surface in non-contactingmanner and connecting the probe to a DC electrometer. This technique ofmeasurement is particularly applicable to the xerography art. The probecomprises a capacitance type of detector and, provided certain criteriaare fulfilled, virtually the entire voltage of the surface undermeasurement can be made to appear at the detector input.

Assuming the use of a capacitance type of detector, the detector willhave a voltage induced at its input which is determined by capacitancevoltage division formulae. In the following description:

C 115 probe-to-surface capacitance,

C total input capacitance to detector, and

C total series capacitance of C and C The relationship between thevoltage at the surface under measurement (V the voltage associated withthe probe-to-surface capacitance (V and the voltage at the detectorinput (V is:

VS: Vps VD.

Therefore, the following relationship exists:

It is apparent that if the capacitance of the detector C is made verysmall compared to the capacitance value C of the probe-to-surfacevirtually the entire voltage at the surface under measurement wouldappear at the detector input. Consequently, the detected voltage wouldbe practically independent of the probeto-surface spacing. A practicalrange of probe-to-surface spacing would normally provide aprobe-to-surface capacitance value C of l to picofarads. However, thetypical input capacitance of a practical open circuited electrometernormally would exceed 10 picofarads. Consequently the total inputcapacitance C to the detector would be relatively high, and the abovediscussed measurement technique would not be practical unless theeffective input capacitance of the detector is reduced by some means.

It is possible to reduce the effective capacitance of the capacitordetector input by feedback means wherein voltage feedback isaccomplished by a voltage follower whose output bootstraps physicalcapacitance. This may be described with relation to FIG. 1 which shows acircuit to reduce the capacitance of coaxial cable 1. Amplifier All isconnected to function as a unity gain voltage follower and forillustrative purposes is assumed to have a gain of +0.9. The shield ofthe cable is connected to the output of amplifier Al as shown in thefigure. Under these conditions, the physical capacitance of the cable isreduced to only 10 percent of its initial value (in the absence of thedescribed feedback connection between the output of amplifier Al and theshield of the cable). If the voltage follower AI had an even higher gainof, for example, 4 0.999, only 0.1 percent of the physical capacitanceof the cable would be effective as a load on the input source. Thisbootstrapping technique can also be used to reduce the effective inputcapacitance to the voltage follower Al itself with the result that inputimpedances may be reduced to practical values wherein R lO ohms and C,O.0lpf. Further these values can be obtained through the use ofconventional components.

- Under such circumstances, if a probe having a capacitance valuevarying from one to ten picofarads is connected to voltage follower A1the capacitance voltage divider error never exceeds 1 percent and auseful device results.

A practical limitation of the described device is that it is not capableof stable operation to DC but rather is a dynamic device capable ofaccurate measurements of change of voltage. This lack of DC measurementcapability can be circumvented if it is possible to periodically scan asurface of known potential. A DC restorer can thus be employed toreference all known voltages to this known voltage.

Consider the case, for example, where all measurements to be made are ofonly one polarity and the probe periodically scans a grounded surface.With reference to FIG. 2a, voltage follower A2 has a closed loop gain ofO.9999 and its input resistor R1 is 10 ohms. Resistor R1 is bootstrappedby capacitor CI (0.1 uf.) and resistor R2 10 ohms) thus markedlyincreasing the effective value of resistor R1 in the useful frequencyspectrum.

All input wiring is shielded and the shield is driven by the amplifieroutput as shown. The input electrode thus has coupling capacitance tothe surface of drum 2 under test but negligible capacitance otherwisebecause all stray capacitance is minimized by feedback via the drivenshield. The surface of drum 2 is assumed to be uniformly positivelycharged, except for the grounded band portion.

The output of voltage follower A2 is coupled to the illustrated DCrestorer circuit, and the output voltage is equal to the DC voltage ofthe charged surface. Through conventional control means (not shown)switch S1 shorts capacitor C2 to ground while the probe scans thegrounded band on the drum surface and switch S2 couples capacitor C2 tocapacitor C3 while the positively charged surface under test is beingsampled. The more general case would be to connect switch S1 andcapacitor C3, as well as the grounded band of drum 2, to a referencepotential having a magnitude which equals or exceeds the maximum valuefor the unknown which can be assumed for the chosen reference potentialpolarity. This also applies to the circuit of FIG. 2b. When switch S2 isopen, capacitor C3 stores the last value until switch S2 closes again.For

the case illustrated, the desired output signal is the voltage across acapacitor C3. If a significant load on capacitor C3 is anticipated, anisolation amplifier is necessary.

Both switches S1 and S2 may be replaced by diodes if desired as shown inFIG. 2b wherein they are respectively replaced by diodes D1 and D2. Thencapacitor C3 would be much smaller than capacitor-C2. The describedtechnique illustrated by FIGS. 2a and 2b is similarly useful where it isknown what the maximum limit'of one polarity is, but where bothpolarities of unknown voltage could exist.

FIG. 3 illustrates the previously described principle in a practicalembodiment. For reasons of superior reliability and performance, it isdesirable to utilize all solid state components in the design of thiscircuit. To achieve the desired performance, the utilization of fieldeffect transistors (FET) is indicated in FIG. 5. The usable inputcircuit components have typical maximum voltage ratings of only a smallfraction of the intended input range. This problem is solved, however,by the technique indicated in the block diagram of FIG. 3. V

The ultra high impedance (UHZ) low level voltage follower A3 is a directcoupled voltage follower with an overload protection circuit covered inapplicants copending application's Ser. No. 759,913, now US. Pat. No.3,61 1,127 entitled Electrostatic Potential and Field MeasurementApparatus, and Ser. No. 759,914 now US. Pat. No. 3,586,91 l entitledOverload Protection Circuit For Amplifier, both filed Sept. 16, I968.

Voltage follower A3 displays a typical open loop input impedance ofapproximately ohms paralleled by less than Spf. It has an open loop gainin excess of 10,000 and therefore displays a closed loop gain of+O.9999. In the closed loop condition its input impedance is dictatedprimarily by external circuit considerations. Voltage follower A3 has alinear operating range of only a few volts and therefore cannot functionby itself and be useful over the desired rangeof several hundred volts.

To achieve such performance, this entire circuit including its powersupply is bootstrapped by a second high level voltage follower A4 whichhas the desired linear output range but has much too low an inputimpedance. This is shown in FIG. 3 where low voltage follower A3 feedshigh level voltage follower A4 and the output of the latter drives thecommon line of the floating bootstrapped power supply 3 associated withvoltage follower A3.

High level voltage follower A4 has a closed loop gain +O.9999 and aclosed loop input inpedance of 10 ohms. The output of voltage followerA4 drives peakto-peak detector 12, to produce an output voltage whoseamplitude is equal to the voltage of the surface under measurement. Anelectrometer may be connected to the output of the peak-to-peak detector12 or the DC restorer and detector circuits of FIGS. and 2b, if loadingis a problem.

FIG. 3 shows that the entire voltage follower A3 and its cable and inputprobe are shielded and this shield is driven by the output of voltagefollower A3. By doing so, the effective capcitance is driven to anextremely small value of less than 10 times the real capacitance. Iftherefore a lOOpf. physical total capacitance is driven in this fashion,the resultant capacitance is less than 0.01pf. This meets therequirement for negligible input capacitance even in the presence of areasonably long shielded lead.

As previously discussed, it is desirable to achieve as large acapacitance as possible between the sensitive electrode and the surfaceunder test (cgp) consistent with adequate resolution (dictating a smalldiameter sensitive electrode) and voltage breakdown (dictating a minimumprobe-to-surface spacing). It is similarly necessary that thecapacitance between the sensitive electrode and all other objects orcircuits be negligible. This is achieved by surrounding the sensitiveelectrode by a driven shield as described above. One way to achieve thisphysically is shown in FIG. 4, which discloses the probe shown in FIG. 3in greater detail.

The probe illustrated comprises a conductive copper cylinder 4 havingends 5 and 6 connected to the driven shield. Ends 5 and 6 may beelectrically connected to cylinder 4 by solder connections 8. The bottomof the probe includes circular sensitive electrode 7 insulated from andguarded by the outer ring-shaped electrode formed by end S, which alongwith the previously .described cylinder, comprises part of the totallyshielded environment for voltage follower A3. The sensitive electrodemay be formed by etching a clear ringlike area to completely surroundit, thereby separating and insulating it from the outer electrode formedby end 5. i I

The inner conductor 9 of the cable is connected between the sensitiveelectrode and the input of the voltage follower A3 and is separated fromouter conductor 10 by dielectric 11. The outer conductor 10 as well ascopper cylinder 4 including ends 5 and 6 are driven by the output ofvoltage follower A3 as shown in FIG. 3.

FIG. 5 shows a preamplifier connected to function as an ultra highimpedance low level voltage follower (A3) having unity gain which may beutilized according to the invention, which is substantially disclosed inapplicants copending application Ser. No. 759,813, more fully identifiedabove.

Resistor R1 is a protective resistor functioning to prevent destructionof the input FET T2, which could occur should the gate of F ET T2 bedirectly coupled to the sensitive electrode. Otherwise the gate input ofF ET T2 would effectively contact the high voltage sensitive electrode,which in turn could possibly directly contact high voltage.

Transistor T1 comprises an NPN transistor connected to have zener diodecharacteristics between the input and output source of PET T2. A zenerdiode may alternatively be substituted for transistor T1 as described incopending application Ser. No. 759,914, more fully identified above. Thecommon connection of the base and collector of transistor T1 isconnected to the gate of F ET T2, and the emitter of transistor T1 isconnected to the series connection of one plate of capacitor C4 andsource S of FET T2. The other plate of capacitor C4 is connected throughresistor R3 to the series connection of resistors R2 and R4. Resistor R4is connected to the high voltage supply +l-IV through resistor R13.Since voltage follower A3 has a gain that exceeds +0999), its effectiveconductance and capacitance is reduced to a negligible value. FET T2having a gate, a source S, and a drain D comprises the active inputelement of the voltage follower.

The output of voltage follower A3 is' limited during overload conditionsby transistor T1. If the voltage follower is overloaded, transistor T1conducts in either forward or reverse biased manner, depending upon thepolarity of the input overload. When such conduction occurs, the FET(T2) gate and output circuits are limited to a potential lower than theFET (T2) destruction potential. Transistor T1 connected therebyfunctions as a protective circuit to prevent destruction of F ET T2under overload conditions.

Transistor T1 connected as a diode, exhibits a typical resistance ofgreater than 1,000 megohms and a typical capacitance of less thanpicofarads, and is shown connected between the input and the output ofthe voltage follower A3. It thus exhibits a loading effect on thevoltage follower source which is reduced by a factor of greater than10,000 by feedback, and thus becomes an effective load under normaloperation conditions of greater than 10 ohms in parallel with lessthan0.00lpf. Its normal loading effect is therefore negligible.

FET transistor T2 is an N channel junction FET. Resistors R2, R4, R13and R14 bias its gate in conventional manner. Resistor R2 particularlyfunctions as a gate leak resistor, which is bootstrapped to an extremelyhigh value by the series connection of resistor R3 and capacitor C4connected to the. voltage follower output in source S of PET T2.

Drain D of PET transistor T2 feeds the base of PNP transistor T3 anddrain load resistor R5. The collector of NPN transistor T4, a currentsource, is connected to the collector of transistor T3 and functions asa load for I the latter. The collectors of transistors T3 and T4 and thebase of PNP transistor T6 have a common connection. The load fortransistor T3 is thus the collector of transistor T4, and the base oftransistor T6. The latter is connected as an output emitter follower.The parallel combination of transistors T4 and T6 connected thereby totransistor T3 provides an extremely high dynamic load impedance andtherefore high second stage gain, considering F ET T2 as comprising thefirst stage.

The series connection of capacitor C5 and resistor R10 is connectedbetween the common connection of the collectors of transistors T3 and T4and base of transistor T6 and the floating bootstrapped power supply. Itfunctions to stabilize the low level voltage follower in the fedbackmode.

The collector of PNP transistor T5 is connected to the emitter oftransistor T6 via zener diode CR1, and functions as a constant currentload for the latter. The collector of transistor T5 is also connected tothe emitter of transistor T3 and to resistor R5.

A connection is made from the output at the emitter of transistor T6 tosource S of PET T2 to convert the preamplifier into a precision ACvoltage follower having direct coupled feedback. This insures stabilityof the DC operating biases of the voltage follower. The emitter oftransistor T6 is also connected to the driven shield.

The input capacitance of the preamplifier should be as small aspracticable as previously explained. It is desirable therefore tomarkedly reduce the input capacitance effect on FET T2. The gate-sourcecapacitance thereof may be reduced to an extremely small value byclosing the feedback loop causing the source to precisely follow thegate. The gate-"drain capacitance however would normally remainexcessively large.

To solve this problem, an FET drain bootstrapping circuit is employed.Thus the gate-drain capacitance may also be reduced by introducingcomparable feedback. To provide such feedback, zener diode CR1 isconnected between the emitter of output emitter follower transistor T6and the constant current load. The cathode end of zener diode CR1functions as the power source for drain D of PET T2 and the emitter oftransistor T3 of the second stage. It thus bootstraps drain D of PET T2to provide the desired reduction in gate-drain capacitance.

It is desirable also that the distributed capacitance of the circuit beas low as practicable. The distributed capacitance may be decreased bysurrounding the preamplifier with a conductive shield which may comprisea coating of conductive paint such as silver connected tothe'preamplifier voltage follower output.

Resistors R6-R9 and Rll-R12 serve conventional biasing functions and arethus not described in detail.

The schematic diagram of FIG. 5 indicates one practical physicalconfiguration for the block diagram of FIG. 3, but the invention is notlimited thereto. Voltage follower A3 employs transistor T2 as its inputF ET. lts drain feeds transistor T3 whose collector load is transistorT4, a constant current load. The collectors of transistor T3 and T4 feedtransistor T6, an emitter follower. Transistor T5 functions as aconstant current load for transistor T6 and is connected thereto viaCR1, a zener diode. Note the feedback loop is completed by theconnection between the emitter of transistor T6 and the source of PETT2, thus converting the amplifier into a voltage follower. The cathodeof zener diode CR1 feeds transistor T3 and T2, thus in essencebootstrapping both the source and drain of PET T2 to reduce itseffective input capacitance to a very low value.

Transistor T1 is connected between the gate and I source of F ET T2. Asthus connected, input-to-output around the voltage follower A3, itseffective leakage and capacitance is reduced by the voltage followeraction as previously described. T1 is a transistor chosen for its smallgeometry and low leakage to function as a zener diode which will conducteither as a forward biased diode or in the reverse breakdown mode in theevent of an overvoltage at the amplifier input which the amplifier isincapable of following. It thus protects the amplifier.

The output of voltage follower A3 feeds transistor T7 and T8 whichconstitute high level voltage follower A4. Transistor T7 is connected asan emitter follower while transistor T8 is a constant current load foremitter follower T7. This constant current load includes zener diodeCR3, the zener source for the base of transistor T8, and resistor R14,the current determining resistor.

Zener diode CR2 is connected between the emitter of transistor T7 andthe collector of transistor T8. The

anode of zener diode CR2 is also connected to and thus feeds the commonline for the floating bootstrapped The electrostatic shield in the powertransformer is connected to the cathode of zener diode CR2 and feeds thefloating bootstrapped power supply therethrough. As it is driven byvoltage follower A3, the capcitance of this power supply to ground andline is reduced to a very small value. Resistor R16 functions as anoutput overload protection device to prevent destruction of transistorT7. Resistors R13 and R14 serve both to supply operating current forzener diode CR 3 and to bias the input circuitry. The ratio of resistorsR13 to R14 is dependent upon the waveform of the input signal and mustbe optimized if the full linear range of the voltage follower is to beused. Resistor R17 and capacitor C6 serve to stabilize the closed loopsystem.

To be useful, voltage follower A4 must be capable of following aperiodic input whose period can be several seconds long. As the inputcapacitance can be of the order of l pf., this would normally dictate aninput resister of the order of ohms. This is quite impractical both fromthe standpoint of availability and because the distributed capacitanceof a single resistance to ground would render the system useless.

The high effective input resistance can be achieved by feedback whilesimultaneously stabilizing the input operating point. The followingtabulation lists practicable values of related components: 7

R2 10,000M ohms R3 0.1M ohms R4 100M ohms C1 lOpf.

Resistor R2 could be driven to the highest practicable value if resistorR3 were omitted. However, the transient performance of the system, inthis case, would verge upon sustained oscillation and would be unusable.Resistor R3 functions to dampen this oscillation to a tolerable value.Resistor R1 is solely for input circuit protection in the event of anexcessive input voltage.

Floating bootstrapped power'supply 3 is connected to the line source ofpower through a transformer as conventionally known. Power supply 3rectifies the applied AC to supply the required DC power supply forvoltage follower A3.

The output of voltage follower A4 is connected to the input ofpeak-to-peak detector 12 which detects the output thereof, and producesan output voltage whose amplitude is equal to the voltage of the surfaceunder measurement. The disclosed voltage follower A4 is one circuit ofthe type that may be employed and other equivalent circuits may besubstituted therefor without departing from the scope of the invention.

1 claim:

1. A wide band voltage follower circuit for deriving an output voltagewhose amplitude is equal to the amplitude of the voltage of anelectrostatically charged surface under measurement comprising:

a probe having a capacitor detector arranged in spaced andnon-contacting manner with the surface under measurement to detect theunknown surface voltage thereof,

a cable connected to the output of the capacitor detector,

a unity gain amplifier connected as an ultra high impedance low levelvoltage follower having an input and output, its input being connecteddirectly to the capacitor detector output through the cable,

first electrostatic shield means driven by the output of the ultra highimpedance low level voltage follower, shielding the latter, the cable,and the probe to minimize the effective input capacitance of the ultrahigh impedance low level voltage follower,

a high level voltage follower having an input and output, its inputbeing connected to the output of the ultra high impedance low levelvoltage follower,

a floating DC power supply having a DC supply output and a common lineconnected to power the ultra high impedance low level voltage follower,bootstrapped with the latter by the high level voltage follower throughthe connection of the common line of the floating power supply to theoutput of the high level voltage follower, and

first means connected to the output of the high level voltage followerto derive the output voltage.

2. A wide band voltage follower as recited in claim 1 wherein the firstmeans is a peak-to-peak detector.

3. A wide band voltage follower circuit as recited in claim 2 furthercomprising:

a reference band region on the surface under measurement,

biasing means connected to bias the peak-to-peak detector and referenceband region to a reference voltage whose amplitude is equal to orgreater than the maximum anticipated voltage of the derived outputvoltage that can be assumed for a given reference voltage polarity.

4. A wide band voltage follower as recited in claim 2 furthercomprising:

electrometer means connected to the output of the peak-to-peak detectorto provide a measurement of the unknown surface voltage.

5. A wide band amplifier as recited in claim 1 further comprising:

an AC power source connected through transformer means to power thefloating DC power supply, and

second electrostatic shield means driven by the high level voltagefollower output and shielding the transformer to minimize thecapacitance of the AC power source to ground and line.

6. A wide band voltage follower circuit as recited in claim 5 whereinthe detector is a peak-to-peak detec- U31.

7. A wide band voltage follower circuit as recited in claim 6 furthercomprising:

a reference band region on the surface under measurement,

biasing means connected to bias the peak-to-peak detector and referenceband region to a reference voltage whose amplitude is equal to orgreater than the maximum anticipated voltage of the derived outputvoltage that can be assumed for a given reference voltage polarity.

8. A wide band voltage follower as recited in claim 6 furthercomprising:

electrometer means connected to the output of the peak-to-peak detectorto provide a measurement of the unknown surface voltage. 9, The wideband voltage follower as recited in claim 4 further comprising feedbackmeans connected between the output and input of the ultra high impedancelow-level voltage follower to provide high effective input resistanceand stabilize the input operatmg point.

1. A wide band voltage follower circuit for deriving an output voltagewhose amplitude is equal to the amplitude of the voltage of anelectrostatically charged surface under measurement comprising: a probehaving a capacitor detector arranged in spaced and noncontacting mannerwith the surface under measurement to detect the unknown surface voltagethereof, a cable connected to the output of the capacitor detector, aunity gain amplifier connected as an ultra high impedance low levelvoltage follower having an input and output, its input being connecteddirectly to the capacitor detector output through the cable, firstelectrostatic shield means driven by the output of the ultra highimpedance low level voltage follower, shielding the latter, the cable,and the probe to minimize the effective input capacitance of the ultrahigh impedance low level voltage follower, a high level voltage followerhaving an input and output, its input being connected to the output ofthe ultra high impedance low level voltage follower, a floating DC powersupply having a DC supply output and a common line connected to powerthe ultra high impedance low level voltage follower, bootstrapped withthe latter by the high level voltage follower through the connection ofthe common line of the floating power supply to the output of the highlevel voltage follower, and first means connected to the output of thehigh level voltage follower to derive the output voltage.
 2. A wide bandvoltage follower as recited in claim 1 wherein the first means is apeak-to-peak detector.
 3. A wide band voltage follower circuit asrecited in claim 2 further comprising: a reference band region on thesurface under measurement, biasing means connected to bias thepeak-to-peak detector and reference band region to a reference voltagewhose amplitude is equal to or greater than the maximum anticipatedvoltage of the derived output voltage that can be assumed for a givenreference voltage polarity.
 4. A wide band voltage follower as recitedin claim 2 further comprising: electrometer means connected to theoutput of the peak-to-peak detector to provide a measurement of theunknown surface voltage.
 5. A wide band amplifier as recited in claim 1further comprising: an AC power source connected through transformermeans to power the floating DC power supply, and second electrostaticshield means driven by the high level voltage follower output andshielding the transformer to minimize the capacitance of the AC powersource to ground and line.
 6. A wide band voltage follower circuit asrecited in claim 5 wherein the detector is a peak-to-peak detector.
 7. Awide band voltage follower circuit as recited in claim 6 furthercomprising: a reference band region on the surface under measurement,biasing means connected to bias the peak-to-peak detector and referenceband region to a reference voltage whose amplitude is equal to orgreater than the maximum anticipated voltage of the derived outputvoltage that can be assumed for a given reference voltage polarity.
 8. Awide band voltage follower as recited in claim 6 further comprising:electrometer means connected to the output of the peak-to-peak detectorto provide a measurement of the unknown surface voltage.
 9. The wideband voltage follower as recited in claim 4 further comprising feedbackmeans connected between the output and input of the ultra high impedancelow-level voltage follower to provide high effective input resistanceand stabilize the input operating point.
 10. The wide band amplifier asrecited in claim 5 further comprising protection means connected to theinput of the ultra high impedance low-level voltage follower to preventdamage thereto in the event of excessive input voltage.